1. Technical Field
The present disclosure relates to the reduction of temperature-caused degradation in the performance of a digital reader. More specifically, the present disclosure relates to reducing the temperature-caused degradation in the performance of a digital reader, where the digital reader has an electronic display containing pixels. The reduction in performance degradation is obtained by positioning at least one sheet of compressed particles of exfoliated graphite in thermal contact with a plurality of the pixels of the electronic display in the digital reader.
2. Background Art
Digital readers are an exciting new branch of technology, allowing a user to access voluminous materials using a relatively compact and portable device. The most well-known of these devices is the Amazon Kindle™ digital reader, which professes to be able to store over 1000 books. Another device gaining popularity is the Sony e-Reader™ digital display. Digital readers each use one of several different technologies in its visual display. These technologies are sometimes referred to as electronic paper or e-paper displays.
The most common type of e-paper display technology is referred to as an electrophoretic display (EPD), and is available from companies such as E Ink Corporation and SiPix Imaging, Inc. In EPD, pixels change color in response to a change in the charge. The pixels can change from a “resting” color to a dye color, or swap between different color particles. A viscous fluid in the pixels holds the particles in their position without the use of power; thus, EPDs are bistable, that is, they are stable in either of their two positions when no power is applied, and only use power when the pixels are changed from one state to another (i.e. equivalent to “turning the page”). EPDs utilize reflective, non-emissive display technologies (i.e., they rely on ambient light or lighting from the front of the display for viewing, are not backlit, and the pixels do not emit light themselves) and because of this and EPD's bistable nature, do not use a great deal of power as compared with traditional emissive display technologies like plasma, backlit liquid crystal display (LCD), or organic light emitting diodes (OLED).
One approach for an EPD is to use spheres having two colors, which are suspended in a viscous liquid between rubbery sheets. As a charge is applied, the sphere rolls into a first position (representing one of the two colors) and then stays in place after the charge is removed. When the charge is applied again, the sphere rolls into a second position (representing the second of the two colors) and then stays in place after the charge is removed. In this way, controlling which of the pixels is exposed to the charge will control the image displayed.
Other e-paper technologies include electronic liquid powder (ELP) or quick response liquid powder display (QR-LPD), in which particles are suspended in air. The particles flow as in a particulate suspension, making this technology very sensitive to electricity and thereby fostering fast reaction. In addition, ELP and QR-LPD displays can reduce image distortion when the display medium is bent or flexed.
Other types of e-paper technologies are under development. In electrowetting displays (EWDs), water droplets on a hydrophobic surface react differently to the application of a charge. Electrochromic (EC) displays include EC display cells which are built up from the combination of a conducting polymer coated paper, a printed electrochromic polymer film, a printed electrolyte pattern and a protective seal layer. In the resulting display cell, the optical contrast is a result of the contrast between the white paper surface and the electrochromic materials switched to its colored state. Cholesteric liquid crystal displays (ChLCDs) and bistable nematic LCDs are two additional technologies being developed for digital readers. Glass-based display using micro-electro-mechanical system (MEMS) technology utilize a reflective technology called IMOD (Interferometric MODulation), with MEMS structures at its core; they use light interference for color generation. Photonic Crystals (P-Ink) are small artificial opals which can change color by electrical stimulation. These opals are integrated into a layer of millions of tiny spheres, which are embedded into an electroactive polymer. By applying a controlled current, the crystals can be maneuvered to produce the entire light spectrum.
Each of the e-paper display systems disclosed herein can be generally defined as a non-emissive display which utilizes a plurality of fluid-containing pixels, with many of these e-paper display systems also being bistable, which require no power to maintain them in either of their two states. Additionally, for a high resolution e-paper display, a thin film transistor (TFT) backplane is typically necessary to drive the pixels. A TFT is a specific type of field-effect transistor produced by depositing thin films of a semiconductor active layer as well as a dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays, but the substrate may also be plastic, lending itself ultimately toward flexible e-paper displays.
One issue facing e-paper displays is exposure to temperature extremes, whether high temperatures, especially temperatures in excess of 35° C., or low temperatures, especially temperatures below 20° C. These temperatures can be experienced if, for example, the digital reader containing the e-paper display is left in a vehicle on a hot or cold day, respectively. High temperatures can cause image ghosting, which occurs when an image is retained even after the display is changed. Low temperatures can lead to delayed response time for the display. In addition, as more functionality is added to digital readers, such as internet connectivity, other heat sources are placed in close proximity to the display and thermal issues are expected to worsen. While the reason for these thermally-induced effects is not fully understood, one theory is that the extremes in temperature lead to changes in fluid viscosity, and the changes in fluid viscosity in turn cause the disadvantageous effects; another theory is that the TFTs themselves are very temperature sensitive, leading to noticeable visual artifacts when non-uniform temperature gradients are induced on the e-paper display.
Accordingly, what is sought is a method for avoiding or reducing the temperature-caused degradation in the performance of an e-paper display in a digital reader, whether it takes the form of delayed response or ghosting, or other thermally-induced issues, and a digital reader for which such degradation is reduced.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to commercially as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.